This invention relates to new and improved apparatus for processing electrical signal and, in particular, to a photo-acoustic space-time processor for multiple time-varying signals such as received by a sonar or radar system utilizing a plurality of signal receiving elements formed as an array.
Array radar and sonar systems detect incoming signals by means of receptor elements which are suitably spaced with respect to one another to form an array in a manner well known in the art. The relative amplitudes and phases of incoming signals received at each of the receptor elements of the array are processed both to establish a desired radiation pattern or beam and to extract the desired intelligence signal from the received signals in the direction of the beam. Processing the received signals is effected by detecting and combining the individual element sonar or radar signals so as to determine beam direction from their relative phase relationships and to obtain the intelligence signal from their time variations in amplitude and frequency.
A typical linear array will produce a radiation pattern or beam which is of fan shape; a two-dimensional array commonly yields a beam that has a conical shape. The beam may be directed or steered either by physically changing the orientation of the array or by electronically varying the relative phases of the signals detected by the elements of the array. The electronic method usually is preferred and, in the past, has normally been accomplished by differentially varying the phase delay of the signals detected by the antenna array elements with one or more phase shifters.
The range and reliability of an array radar or sonar can be improved by increasing the number of receiving elements. However, additional receiving elements require additional phase shifters and supporting electronic controls so that as the system becomes more sophisticated, the associated electronic circuitry expands and becomes correspondingly more complex. The subject invention overcomes such disadvantages by providing a single photo-acoustic space-time signal processor for beamforming and steering all the elements of the array, thus reducing the per-element cost and complexity of the array processor.
It is well known in the prior art that light signals may be passed through an acoustic medium and modulated by an interacting sound wave. One such device is described in U.S. Pat. No. 3,111,666 (R. M. Wilmotte, Nov. 19, 1963). In that patent, a reference light signal is passed sequentially through two birefringent acoustic delay lines. The first delay line is excited by a signal representative of the transmitted pulse and the second delay line is excited by a signal representative of the echo pulse. At some point along the delay line, the two signals will correlate and light intensity will be a maximum. The distance along the delay line at which correlation occurs is sensed by a video receiver and provides a measure of the range of the target. Other acousto-optical signal processors are described in U.S. Pat. No. 3,421,003 (G. J. Pratt, Jan. 7, 1969) and U.S. Pat. No. 3,634,749 (Robert M. Montgomery, Jan. 11, 1972). However, in each of the foregoing cited patents, the light signal is an unmodulated reference signal and no means are provided for simultaneously space and time processing the received signal nor for processing multiple signals received from an array as is accomplished by the present invention.
It is also well known in the art to use a method and apparatus for directly converting an optical image into an electrical signal representative of the spatial Fourier transform of the image by interacting the optical image with sound waves (U.S. Pat. No. 3,836,712-Kornreich et al). This method and apparatus is termed a "direct electronic Fourier transform", sometimes referred to by the acronym DEFT. It relies on the recognition that certain media have special properties that permit deriving electrical signals representative of the optical image incident on the media. By projecting an optical image onto such a medium, and then inducing an acoustic surface wave or bulk wave of given acoustic frequency in the medium, it is possible to derive a signal which is representative of the spatial Fourier transform of the optical image for that frequency. By sweeping the acoustic frequencies through a desired bandwidth, the entire optical image may be converted into a series of Fourier transforms.
It is an object of this invention to provide a new and improved device and a method for the simultaneous space and time processing of electrical signals such as radar or sonar signals.
It is another object of this invention to provide a new and improved device and method for forming and scanning sonar and radar beams.
It is a further object of this invention to provide a new and improved means and method for photoacoustic processing of radar, sonar and other signals of time-varying phase-related character.